1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
When optical scanning with a high printing dot density is conducted by an optical scanning system with a wide scanning width, it may be necessary to increase the width of an incident beam into a scanning lens (third optical system) in order to provide a small scanning spot diameter corresponding to a dot density. An incident beam width L is generally represented by formula 1:L=4×λ×f/(π×φ)  (formula 1),wherein λ, f, and φ are a used wavelength, the focal length of a scanning lens, and a scanning spot diameter, respectively.
If the wavelength, the focal length of a scanning lens, and the scanning spot diameter are, for example, 660 nm, 500 mm, and 40 μm, respectively, the incident beam width L is 10.5 mm according to the above-described formula 1. Because the aperture diameter of a collimator lens (first optical system) having been used conventionally is commonly about 7 mm and it may be impossible to emit a wide beam of 10.5 mm, a second optical system such as a beam expander for expanding a light beam behind a collimator lens is required.
The second optical system may be essential for an optical scanning device with plural light sources, because the effect of crossing beams from plural light sources at the position of a rotary polygon mirror is also provided. If this second optical system is not provided, the space between plural light beams having passed through a first optical system is increased and has a width larger than the width of a single beam at the position of a rotary polygon mirror. In that case, it may be impossible to scan a required scanning range with sufficient amount of light unless the inscribed circle radius of the rotary polygon mirror is increased, and there may be a limit to increasing of the inscribed circle radius of the rotary polygon mirror, which may be problematic.
Meanwhile, when optical scanning is conducted with a high printing dot density, there may be a limit to the rotational frequencies of light-deflecting means such as a rotary polygon mirror, and therefore, it may be necessary to increase the number of light sources. The rotational frequency R of a rotary polygon mirror is represented by formula 2:R=D×V/(n×m)  (formula 2),wherein D, V, m, and n are a printing dot density, a process speed, the number of mirror planes of light-deflecting means, and the number of plural beams, respectively.
If the printing dot density, the process speed, the number of mirror planes of light-deflecting means, and the number of beams are 1200 dpi, 70 inches/second, 8, and 20, respectively, the rotational frequency R is 31500 revolutions per minute, which is a practical rotational frequency.
Furthermore, when optical scanning is conducted with a high printing dot density, there may be a limit to bema modulation, and therefore, it may be increase the number of light sources. A time period per 1 dot, T, is represented by formula 3:T=n×m/(F×D×D×V)  (formula 3),wherein D, V, f, m, and n are a printing dot density, a process speed, the focal length of a scanning lens (third optical system), the number of mirror planes of light-deflecting means, and the number of beams, respectively.
If the printing dot density, the process speed, the focal length of a scanning lens (third optical system), the number of mirror planes of light-deflecting means, and the number of beams are, for example, 1200 dpi, 70 inches/second, 500 mm, 8, and 40, respectively, the time period per 1 dot, T, is 12.8 ns, which is a modulatable value. Therefore, it may be required that the number of beams is 40 or more in the specification of the above-described example.
Next, in regard to arrangement of light-emitting elements, for example, 40 light-emitting elements are arranged in a line and their arrangement angles are changed, whereby it may be possible to adjust a scanning interval, which may be preferable. Herein, if the space between the light-emitting elements is, for example, 20 μm, the length of the arrangement is 780 μm and the image height of a light source in the first optical system which is up to about ±0.4 mm will be used.
Conventionally, when the printing dot density is, for example, 600 dpi for the same specification as the above-described example, it may have been sufficient for the number of beams to be 10 due to the restriction on the time period per 1 dot, T, and therefore, it may have been sufficient for the image height of a light source to be about ±0.1 mm. In this case, if the focal length of the first optical system is, for example, 17 mm, the angle of emission from the first optical system of a light source at the end thereof is 0.337 degrees and a light beam from the light source passes comparatively near the optical axis thereof, so that aberration caused by the first and second (lens systems for shaping a light beam) and third (scanning lens system for imaging on a medium to be scanned) optical systems may be sufficiently small and may not be problematic.
However, if the image height of a used light source is 4 times, that is, ±0.4 mm and the focal length of the first optical system is 17 mm similarly to the above-described example, the angle of emission of a light-emitting element at the end thereof is 1.35 degrees and a light beam from the light-emitting element is displaced from the optical axis thereof, so that lens aberrations cased by the first, second and third optical systems, in particular, an image surface deviation may be problematic.
In order to solve it, it may be necessary to correct for lenses constituting the second and third optical systems. However, some characteristics such as a constant speed of scanning and an image position for each scanning position are taken into consideration for a set of the second and third optical systems, and therefore, it may not be easy to include an item for improving the characteristics of an image surface deviation associated with the image height of a light source. Furthermore, when correction is conducted by an aspheric lens in the second and third optical systems, the size of the lens may be so large that it may be difficult to realize an aspheric lens or a large cost increase may be involved for its realization, which may not be practical.
For a solution concerning the above-described aberrations, for example, in Japanese Patent Application Publication No. 05-273463, a single aspheric and glass lens is used as a collimator lens that is a first optical system and combined with an optical scanning and imaging system that is composed of a plastic only whereby a performance change caused by a temperature change is corrected for.
For a solution concerning the above-described aberrations, for example, in Japanese Patent Application Publication No. 2002-267976, one optical element that has an axial power of about 0 and an aspheric shape is arranged between a coupling lens system and light-deflecting means in order to suppress a spherical aberration generated in the case where the numerical aperture NA of an optical system is large and provide an optical scanning device that is adaptable for attaining a high dot density of a recording medium at a low price.
For a solution concerning the above-described aberrations, for example, in Japanese Patent Application Publication No. 60-121412, a single aspheric lens is provided which has a numerical aperture NA of 0.12-0.2 and both well-corrected spherical aberration and sine condition.
For a solution concerning the above-described aberrations, for example, in Japanese Patent Application Publication No. 63-189822, positive and negative cylindrical lenses are provided between a light source and light-deflecting means so that a spherical aberration possessed by a deflecting lens is corrected for.
As described above, some kinds of single aspheric and glass lens have been suggested conventionally but not all of the problems have been solved. That is, in the suggestions in Japanese Patent Application Publication No. 05-273463, Japanese Patent Application Publication No. 2002-267976, Japanese Patent Application Publication No. 60-121412, and Japanese Patent Application Publication No. 63-189822, as described above, an image surface deviation associated with the image height of a light source has not been taken into consideration and there has been a problem such that the deviation of light spots may be large in the case where a large number of light-emitting elements are arranged in a liner manner and the image height of a light source is large. Furthermore, if such an optical scanning device is used for an image forming apparatus, there may be a disadvantage such that a line width may not be uniform whereby an image may be unstable and degradation of an image quality may be caused.